1. Field of the invention
The present invention concerns digital transmitters for transmitting data between data transmission terminal equipments via a multiservice digital private automatic branch exchange (PABX) which has access to public networks and services; in particular, it has access at 64 kbit/s to the public switched telephone network and in this case is synchronized to the switched network.
2. Description of the prior art
Asynchronous digital transmitters are already in use, connected by a V.24 socket to a terminal equipment and operating at standardized data rates up to 19 200 bit/s. A digital transmitter of this kind transmits data over a digital link which connects it to a PABX in half-duplex asynchronous mode. This is described in an article entitled The multiservice automatic switching unit OPUS 4000 by M. BAUDIN, L. LE, J. R. MEJANE, F. SEVEQUE and P. ANIZAN published in the French Journal Commutation et Transmission No 4 1985. The transmitter and the PABX communicate by means of successive 125 .mu.s frames. Each frame is made up of two messages, one transmitted to the transmitter and the other transmitted by the transmitter. The messages are identically constituted as follows:
one start bit, PA1 one signalling bit for an 8 kbit/s signalling channel, PA1 one data bit for an 8 kbit/s data channel, PA1 eight speech bits for a 64 kbit/s speech channel, PA1 one line balancing parity bit. PA1 a line interface circuit for modulating and demodulating digital signals transmitted on said line, PA1 a multiplexing circuit interfaced to the line interface circuit for multiplexing and demultiplexing signals in the various channels of the line for half-duplex exchange of messages within a 125 .mu.s frame, one of said channels being a signalling channel and another being a 64 kbit/s data channel, PA1 means for producing a frame clock defining said 64 kbit/s channel of the line, PA1 means for processing data and signalling for their transmission in both directions to the PABX over the digital line and to the terminal equipment, in which digital transmitter: PA1 at least a first socket is provided for selective connection to said transmitter of one of the V.24, V.35/V.24, X.21 interfaces of terminal equipments with different standardized data rates, PA1 means are provided for generating a bit clock at a frequency very slightly greater than that of the data from the terminal equipment connected to the transmitter and the rising edges of which control the transmission of data in one direction and the falling edges transmission of data in the other direction between the digital transmitter and the equipment, and PA1 means are provided for synchronizing said bit clock to said frame clock with a synchronization period derived from the data rate of the terminal equipment connected relative to the frame clock frequency, the corresponding edge of the bit clock being resynchronized relative to the frame clock in each synchronization period.
Each message is followed by a guard time made up of eight stop bits for the two messages of a frame.
This form of message allows simultaneous transmission of data and speech in the case of speech and data terminal equipments. As an asynchronous digital transmitter does not transmit speech it uses the 64 kbit/s speech channel to transmit data and the signalling channel for the dialogue between the PABX and the terminal. The start bit of the message sent by the PABX enables recovery of the 8 kHz system clock in the transmitter.
The message data has its data rate changed to match the standardized data rate of the terminal to the data rate of the line which carries the data, in both directions, terminating at the interface circuit of the terminal. This data rate change is effected through under-use of the 64 kbit/s transmission channel of the line and by using framing in accordance with CCITT Recommendation V.110 (equivalent to ECMA standard 102) carried out at the PABX for transmission of messages to a remote terminal equipment via the switched network.
For transmission over the line the messages are modulated with the DC component eliminated: logic "1" is not modulated, while logic "0" is alternately modulated as a positive and a negative level (+3 V, -3 V).
An object of the present invention is a synchronous digital transmitter for synchronous transmission of data between a multiservice digital PABX and various types of terminal equipment with varied standardized data rates up to 64 kbit/s.